Exhaust gas discharged from an internal combustion engine of, for example, an automobile contains toxic components such as hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx). Hitherto, ternary catalysts have been used for removing such toxic components for detoxifying the exhaust gas.
Some ternary catalysts are produced by applying, onto a honeycomb carrier made of, for example, a ceramic or metallic material, a certain combination of a noble metal (e.g., Pt, Pd, or Rh), and alumina, ceria, zirconia, or a composite oxide thereof. For example, there has been proposed an exhaust gas purifying catalyst containing a composite oxide having an apatite-type structure (see, for example, Patent Documents 1 and 2).
However, such a conventional ternary catalyst generally poses problems in that the catalyst does not initiate removal of toxic components contained in exhaust gas until a relatively high temperature, and thus the catalyst may fail to satisfactorily remove the toxic components at a low temperature. For example, such a conventional ternary may fail to satisfactorily remove toxic components contained in exhaust gas discharged from an engine of, for example, an automobile, since the catalyst is in a relatively low temperature state immediately after starting of the engine.
Also, such a conventional ternary catalyst generally poses problems in that the catalyst may exhibit lowered exhaust gas purification performance due to deterioration resulting from use in a relatively high temperature range, and the catalyst may fail to attain consistent exhaust gas purification performance over a low to high temperature range.
Therefore, demand has arisen for a ternary catalyst which attains exhaust gas purification effect in a relatively low temperature state, and which also attains gas purification performance in a high temperature range. The present inventors have previously proposed an exhaust gas purifying catalyst which contains a composite oxide represented by (Laa-xMx) (Si6-yNy)O27-z and a noble metal component that forms a solid solution with the composite oxide or is supported on the composite oxide, which exhibits high low-temperature activity and excellent heat resistance, and which can attain consistent exhaust gas purification performance (see Patent Document 3).